Add Key#decompress_pubkey that return uncompressed public key with he…

…x format

lib/bitcoin/key.rb

@@ -261,6 +261,12 @@ def xonly_pubkey
       pubkey[2..65]
     end
 
+    # Convert this key to decompress key.
+    # @return [String] decompress public key with hex format.
+    def decompress_pubkey
+      pubkey.htb.bytesize == PUBLIC_KEY_SIZE ? pubkey : to_point.to_hex(false)
+    end
+
     # check +pubkey+ (hex) is compress or uncompress pubkey.
     def self.compress_or_uncompress_pubkey?(pubkey)
       p = pubkey.htb

spec/bitcoin/key_spec.rb

@@ -318,4 +318,26 @@ def test_generate
     end
   end
 
+  describe '#decompress_pubkey' do
+    context 'pure ruby' do
+      it 'return decompressed public key' do
+        test_decompress
+      end
+    end
+
+    context 'libsecp256k1', use_secp256k1: true do
+      it 'return decompressed public key' do
+        test_decompress
+      end
+    end
+
+    def test_decompress
+      compress_key = Bitcoin::Key.generate(Bitcoin::Key::TYPES[:compressed])
+      uncompressed_pubkey = Bitcoin::Key.new(priv_key: compress_key.priv_key, key_type: Bitcoin::Key::TYPES[:uncompressed])
+      expect(compress_key.decompress_pubkey).to eq(uncompressed_pubkey.pubkey)
+
+      uncompressed_key = Bitcoin::Key.generate(Bitcoin::Key::TYPES[:uncompressed])
+      expect(uncompressed_key.decompress_pubkey).to eq(uncompressed_key.pubkey)
+    end
+  end
 end